1. Field of the Invention
The present invention relates to semiconductor optical devices.
2. Description of the Related Art
Patent Document 1 (Japanese Unexamined Patent Application Publication No. 62-183406) describes a waveguide-type optical interferometer. This waveguide-type optical interferometer includes a substrate, two optical guides composed of glass or a plastic formed on the substrate, two optical couplers that connect the optical waveguides to each other at different positions, and phase shifters disposed in the optical waveguides between the optical couplers. Each phase shifter includes a heater disposed on the optical waveguide. The optical path length of the optical waveguide is changed by controlling the temperature of the optical waveguide by heating the heater of the phase shifter.
In recent years, optical modulators that modulate light in response to electric signals from outside have become one of the essential components in configuring optical fiber communication systems and optical information processing systems. In particular, a Mach-Zehnder interferometer type optical modulator that uses a waveguide-type optical interferometer described in Patent Document 1 enables high-speed modulation of 40 Gbps or higher. Since Mach-Zehnder interferometer type optical modulators have a low wavelength chirp under high-speed modulation, Mach-Zehnder interferometer type optical modulators can be used for future ultra high-speed, high-capacity optical communication systems. In particular, Mach-Zehnder interferometer type optical modulators composed of semiconductors are small in size, have low power consumption, and can be monolithically integrated with other semiconductor optical devices such as a laser diode through to achieve wider versatility.
One example of a known semiconductor Mach-Zehnder interferometer type optical modulator is an npin-structured modulator. An npin structured Mach-Zehnder interferometer type optical modulator includes a waveguide structure formed by sandwiching a core layer (i layer) composed of an undoped semiconductor with two cladding layers composed of an n-type semiconductor and then inserting a thin p-type semiconductor layer between the core layer and one of the cladding layers. FIGS. 13, 14A, and 14B are diagrams showing one example of an npin-structured Mach-Zehnder interferometer type optical modulator.
As shown in FIG. 13, a Mach-Zehnder interferometer type optical modulator 100 includes two optical waveguides 110 and 120, an input optical coupler 130, an output optical coupler 140, and upper electrodes 150 and 160. These components are formed on an n-type semiconductor substrate 101 (refer to FIGS. 14A and 14B). The optical waveguide 110 includes a waveguiding section 211, a phase shifting section 212, and a waveguiding section 213, aligned in that order in the waveguiding direction. The optical waveguide 120 includes a waveguiding section 121, a phase shifting section 122, and a waveguiding section 123 also aligned in that order in the waveguiding direction. The phase shifting sections 212 and 122 are sections in which the upper electrodes 150 and 160 are respectively disposed and to which a signal voltage is applied. Each of the optical waveguides 110 and 120 has one end connected to the input optical coupler 130 and the other end connected to the output optical coupler 140.
Referring now to FIGS. 14A and 14B, the Mach-Zehnder interferometer type optical modulator 100 includes an n-type lower cladding layer 103, core layers 104a and 104b, p-type semiconductor layers 105a and 105b, and n-type upper cladding layers 106a and 106b. The core layer 104a is interposed between the n-type lower cladding layer 103 and the n-type upper cladding layer 106a. The p-type semiconductor layer 105a is interposed between the core layer 104a and the n-type upper cladding layer 106a. The core layer 104b is interposed between the n-type lower cladding layer 103 and the n-type upper cladding layer 106b. The p-type semiconductor layer 105b is interposed between the core layer 104b and the n-type upper cladding layer 106b. The upper electrode 150 is disposed on the n-type upper cladding layer 106a. The upper electrode 160 is disposed on the n-type upper cladding layer 106b. A lower electrode 170 is formed on the back of the n-type semiconductor substrate 101.
A part of the n-type lower cladding layer 103, the core layer 104a, the p-type semiconductor layer 105a, and the n-type upper cladding layer 106a form a mesa structure 107a. The mesa structure 107a constitutes the optical waveguide 110. Similarly, another part of the n-type lower cladding layer 103, the core layer 104b, the p-type semiconductor layer 105b, and the n-type upper cladding layer 106b form another mesa structure 107b. The mesa structure 107b constitutes the optical waveguide 120. Side surfaces of the mesa structures 107a and 107b are buried by, for example, a polyimide resin 108.
According to the Mach-Zehnder interferometer type optical modulator 100, the refractive indices of the core layers 104a and 104b can be changed by applying a reverse bias voltage between the lower electrode 170 and the upper electrodes 150 and 160. As a result, the phase of the light guided in the core layers 104a and 104b can be shifted.